WASHINGTON — WHEN a news story disclosed his army's movements, forcing a battle he had hoped to avoid, Union Gen. William Tecumseh Sherman was outraged. ''It's impossible to carry on a war with a free press,'' the Civil War commander bellowed to his staff.
As General Sherman's successors begin one of the largest peacekeeping missions in history, in command of a 60,000-member NATO force in Bosnia, they might be thinking a similar thought: that it's difficult to carry on a peace with a free press.
In the past, Sherman-like suspicions of the press have been fueled by worries that journalists will reveal information that could aid an enemy and endanger the lives of American soldiers.
Today, as peacekeeping becomes more common than war-fighting, it is not the disclosure of battle plans but the intensive scrutiny of day-to-day operations that jeopardizes military missions.
Magnified by the presence of hundreds or thousands of journalists, every casualty or military setback can take a toll on public and congressional support for operations abroad, defense experts say.
One example: the deaths of 18 American soldiers in 1993 that contributed to President Clinton's decision to sharply downsize a US-led peacekeeping mission to relieve famine in Somalia.
In Bosnia, extensive reporting about the struggle of US forces to throw a bridge across the Sava River has already embarrassed US commanders and pushed them to commit more resources toward its completion.
Carried to an extreme, intensive press coverage in an age of instant global communication could limit the ability of the president to use the military as an effective instrument of US foreign policy, some experts believe.
''Press reports can turn public sentiment against military operations and make it more difficult for a president to carry out his mandate,'' says Alvin Snyder, a senior fellow at the Annenberg Washington Program and author of ''Warriors of Disinformation,'' a book on US and Soviet propaganda during the cold war.
Media 'a two-edged sword'
''The media is a two-edged sword,'' adds Brookings Institution defense analyst Lawrence Korb. ''When they report the horrible things happening in countries like Haiti or Bosnia, it puts pressure on the president to do something. When they report on the horrors of a military operation, it puts pressure on the president to get out.''
''The media gets you in, but makes it harder to stay in,'' notes Mr. Korb.
Hoping to make it easier, Mr. Clinton will travel to Bosnia tomorrow for a ceremonial hands-on with US soldiers assigned one of the more difficult peacekeeping missions in history. Details have been kept secret, but Clinton is expected to visit Tuzla, headquarters of the American force in Bosnia, and possibly Belgrade and Zagreb, the Yugoslav and Croatian capitals.
Another military expert points to a ''zero casualty'' mindset on the part of the public, the media, and the nation's political leaders - a reaction, he says, to media coverage, which often fails to place casualties in any meaningful context, and to the experience of the Gulf war, which was fought under such ideal conditions that very few US casualties resulted.
''We have started to persuade ourselves that we can do these things cost-free,'' says Albert Pierce, professor of military strategy at the National War College, in Washington. ''When casualties do begin, we tend to run.''
By contrast, says Mr. Pierce, America's European allies ''tend to think of casualties not cavalierly but as the cost of a major power doing business overseas.''
Recent history provides notable examples of the effect of press coverage on military decisions. Graphic pictures of the destruction that occurred as US planes attacked Iraqi soldiers retreating from Kuwait along the ''highway of death'' in 1990 were one reason the Bush administration cut the Gulf war short, over Pentagon objections.
After the 1993 ''Ranger'' incident in Somalia, public support for the US peacekeeping mission plummeted.
In Bosnia, meanwhile, news of the first American casualty was widely reported, providing ammunition for congressional critics opposed to the deployment of US troops to help guarantee a peace accord signed in December between the warring Bosnian factions.
Media and public-opinion analysts say the impact of television images is greatest when the reasons for US intervention are not clearly articulated and when America's national interests are not well defined. When the opposite is true, it becomes possible to sustain support for US intervention abroad.
''Television images usually have a short shelf life, and their emotional effects can be tempered by reason,'' argues James Hoge, writing in Foreign Affairs magazine. ''But that requires political leadership that constructs supportable policy, explains it, and knows when to stand fast behind it.''
Twenty thousand American soldiers are now being deployed in Bosnia. Nearly 2,000 US and foreign journalists are on hand to cover their every move.
With direct access to events provided by mass press coverage and new communications technologies, the public has an unprecedented opportunity to make its own judgments about events abroad, independent of the views of opinion leaders.
At the same time, the proliferation of news outlets has provided more channels for administration spokesmen to make their case on Bosnia directly to the American public. ''With more direct channels, it helps the administration to speak directly to the public without the filter of the press,'' says Mr. Snyder.
If their explanations to the public are credible and convincing and if they sustain their public relations offensive, adds Mr. Korb, policymakers ''can contend with the media.''
Pentagon and the press
The proliferation of news outlets serves policymakers in another way. Instead of denying or limiting access to the media, as it did during the invasions of Grenada (1983), Panama (1990), and Kuwait (1991), the Pentagon now seeks to expand access for reporters as far as security considerations allow.
The theory: The best insurance against harmful disclosures is to send reporters to the front, where they can gain an understanding of conditions on the ground.
''The feeling in the military is that press censorship is passe because the key is to afford the media an opportunity to go and report on the operation first hand,'' says retired Adm. William Lawrence, co-author of a new report on the media and the military entitled ''America's Team: The Odd Couple.''
But if the policy provides unparalleled access to reporters, like the top-secret briefings provided by senior military officers on the eve of the US occupation of Haiti, it also enables the Pentagon to manage the news in a way that helps sustain public support.
''The new system is right out of a Madison Avenue manual for a publicity blitz,'' writes journalist and author Peter Andrews. ''If you want pictures, you will get more than you can possibly use, but they will be our pictures. If you want quotes, you will get them by the hour, but they will be our quotes.''